Instruments & Cameras

Astronomical Observatory of Belgrade (AOB) provided several cameras and instruments to be able to work on both, the 60cm Cassegrain and the future 1.5m Nasmyth telescopes. These instruments are stated and briefly described below:

- FIELD OF VIEW AT THE CASS. FOCUS WITHOUT FOCUS REDUCER: 15.7' x 15.7'

- FIELD OF VIEW AT THE CASS. FOCUS WITHOUT FOCUS REDUCER: 31.7' x 31.7'

Linearity

The most direct way to test for CCD non-linearity iis to measure the intensity of a stable lamp (for example beta light or stabilized LED) as a function of exposure time and then plot level per unit time versus level. For linear camera, this plot should be constant. Deviation from the constant value is a measure of the non-linearity.

We have measured the linearity of the Apogee ALTA U42 CCD camera in this way. We use the so called Bracketed Exposure method which involves making images with varying exposure time which are bracketed with images with constant exposure time. For instance, in a sequence 5s, 2s, 5s, 7s, 5s, 10s, 5s, 13s, 5s, 17s, 5s, 20s, the images with increasing exposure time are bracketed with 5s exposures. The constant exposures (5s in this example) are used to check and, if exist, to calibrate the variation of the lamp intensity (lamp drift).

Figure bellow shows the equipment we used for the linearity test. Left panel of the figure shows the schema of the device (taken from Berry and Burnell, 2000). Basically, it is a light-box (made from heavy cardboard or light plywood) which consist of a faint source of light on the left (circuitstabilized LED in their case) and CCD camera on the right side of the box. They put two light diffusers into the device - one in front of the camera and the other after the light source (they recommend using an opal-glass or milk plastic as a diffuser). The light-source intensity is varied by changing several slides (made also from heavy cardboard or light plywood) with drilled holes of different size. The light baffle in the middle of the device serves to reduce the scattered light from its inner wall. Right panel of the figure shows our improvised device. The light-box is actually the adapter for the 60cm telescope in our case. The camera and the light source are attached to the adapter on the right and left side of the adapter following the schema. We used semi-transparent paper as a light-diffuser.

Figure below is the result of the test. Panel on the left, shows the light level (corrected for lamp drift) versus exposure time. Solid line is a linear fit to data with counts bellow 30 000 ADU where we assume that our camera is linear (coefficients of the linear fits are given in the legend). Therefore, the values returned by the linear fit are the expected values, that is, the light level that we expect to measure if the camera would be linear. Panel on the right shows the difference between measured and expected light level (in percentage) as a function of light level.

Wa can summarize the results as follows:

(i) an extreme difference can be noticed at low light level (around 2.7% for the first point where the light level is around 2000 ADU after the bias and thermal correction). This effect is a result of charge traps in the image or in the shift register and is sometime referred to as deferred charge. Most CCDs respond nonlinearly at this range,

(ii) beside deferred charge, the exposure time for the first images in both sets are 1.5 second where the shutter effect may have some effect. The large difference we have for the first point is probably a combination of the two effects,

(iii) the difference between measured and expected light level is below 1% upto around 46 000 ADU after the bias and thermal corrections. Therefore, if we need observation with photometric accuracy better than 1%, we must observe in range from 2000 ADU to 46 000 ADU after the bias and thermal correction. Since the average bias level is around 1400 ADU, and the bias level is dominant over the thermal level even for the longest exposure time, this means that we must observe between 3400 ADU and 47 400 ADU measured on raw images,

(iv) the camera is highly non-linear above 58 000 ADU where the difference between the measured and the expected level is larger then 2%. Generally, this range should be avoided when observing or the data should be corrected for non-linearity as will be described later,

(v) according to the linear fit on Fig.5, the time coordinate intercept -0.1s. This means that a time correction of 0.1s should be added to the selected exposure time. This is especially important when observing stars that require very short exposure times. For example, a 1s exposure on such a star would really be exposed 1.1s making the star 0.1 magnitude brighter when compared with stars in longer exposures.

Apogee E47

Apogee E47+ is a smaller format back-illuminated CCD camera than E42 with the following characteristics:

SBIG AO-7 adaptive optics

The AO-7 adaptive optic has been specifically designed to enable an ST-7/8/9/10/2000 camera user to obtain the ultimate in image resolution that a telescope and site can achieve. Figure below shows the AO-7 adaptive optic attached to the ST-10ME CCD camera (Credits: Vince Oliver).

Portable Spectrograph

Astronomical Observatory of Belgrade has one portable fiber-feed spectrograph dedicated for use at the 60cm Cassagrain telescope and the future 1.5m-class telescope at the Astronomical Station of Vidojevica. The spectrograph has been planed for recording low resolution spectra of relatively faint stars and asteroids, medium-resolution spectra of relatively bright stars and studies of the variations in highly broadened spectral line profiles.The spectrograph parameters are as follows: